1. Field of the Invention
The present invention relates to an apparatus for spreading the information signals in a CDMA (Code Division Multiple Access) system supporting various information rates, and more particularly, to an apparatus for selecting a channel-separating and a PN (Pseudo-random Noise) spreading method according to the rate of the symbol to spread.
2. Description of Related Art
In DS-CDMA (Direct sequence CDMA) systems, there are two different channel spreading methods, one of which is to spread the signal with different codes from one another, the other of which is to use one long PN code with different time offsets. The conventional CDMA systems have used one of these methods and a combination thereof.
The conventional CDMA system, as developed by QUALCOMM Incorporated, has used the first method to separate the forward channels in the same cell or sector. In other words, the Sync Channel, the Paging Channels and the Traffic Channels of the forward link are spread and separated by the orthogonal Walsh functions with distinct indexes. In the case of a single-path ideal channel and one cell environment, interference between the channels does not exist because of the orthogonal property of the Walsh functions.
In the real mobile radio channel, there is multiple access interference (MAI) between the channels even though the orthogonal sequences are used because the multipath fading characteristics of the channel lead to distortion of the signal. However, the MAI, which is caused by the fading effect of the channel, is still lower than that in the case of channel-separation with non-orthogonal sequences.
In the conventional CDMA systems, the second method above has been used to identify a forward CDMA channel from the adjacent cells or sectors. PN binary codes are used to distinguish signals received at a mobile station from different base stations. All CDMA signals in the system share a quadrature pair of PN codes. Signals from different cells or sectors are distinguished by time offset from the basic code. This relies on the property of PN codes that the autocorrelation averages to zero for all the time offsets greater than a single code chip time.
The conventional CDMA systems that provide voice-based services have a small number of data rates so it is natural to make the channels to be orthogonal using orthogonal sequences. The channel-separation and PN spreading of the traffic channel in the conventional CDMA systems is shown in FIG. 1. A traffic information bit is input into a traffic channel that passes through a channel encoder, such as a convolutional encoder 11, a symbol repeator 12 and a block interleaver 13, in this order. Thereafter, the output symbols from the block interleaver 13 are spread in an orthogonal spreader 14 by a Walsh function with an index allocated to the channel during the call setup procedure and the output from the orthogonal spreader 14 is spread in the pilot PN spreader 15 and 16 (I channel and Q channel), respectively, by the pilot codes with a time offset which is allocated to the corresponding cell or sector. All the forward channels within one cell or sector share a pair of pilot PN codes with the same time offset.
One of the typical multiple accesses applied to the mobile communication systems is the CDMA communication system. In IS-95, a CDMA common air-interface (CAI) adopted in the United States as a cellular and personal communications service (PCS) standard, the forward channels in one cell or sector are spread by the Walsh functions as channel-separating orthogonal codes and a pair of PN sequences of period 2.sup.15 with a given time offset as pilot PN codes. Therefore, there is a little intra-cell interference at a mobile station because all the forward channels can be spread with orthogonal sequences and transmitted synchronously.
However, in the systems in which the Walsh functions are used, it is difficult to transmit information data with various rates because of the relationship between the symbol rate at the input to the orthogonal spreader and the chip rate at the output of the PN spreader. That is, one symbol at the input to the orthogonal spreader should be spread with one or more periods of the Walsh sequence to guarantee orthogonality at the demodulator of the mobile receiver.
A spreading method, which uses the long codes only for channel-separation without orthogonal spreading, is considered to support a variety of rates of user information. This method gives a lot of flexibility in system design, but there is no orthogonality between the channels even among the channels satisfying the above Walsh condition, which lowers the bit error performance.